Evaluation of quantitative and qualitative antibody responses to Streptococcus pneumoniae and Haemophilus influenzae type b conjugate vaccines The vertical transmission of HIV-1 infection from mother to infant can be reduced by at least two-thirds using simple perinatal anti-retroviral (ARV) drug regimens. Nevertheless, at least 10-12% of children born to HIV-1 infected mothers will still be HIV-1 infected. The management of these children through anti-retroviral treatment (ART) and prophylaxis against common opportunistic pathogens has been successful in developed countries. Implementation of such an intervention in resource-constrained countries will only be possible through novel strategies. Maj or causes of morbidity and mortality in HIV-1 infected children are infection by Pneumocystis carinii and bacteria, specifically Streptococcus pneumoniae and Haemophilus influenzae type b (Hib). Prophylaxis against these pathogens includes co-trirnoxazole and the new conjugate vaccines. This project will allow for an evaluation of the effectiveness of cotrimoxazole prophylaxis in HIV-1 infected children and its impact on selecting for antibiotic-resistant bacteria. Furthermore, the impact of ART on the immunogenicity of the conjugated pneumococcal and Hib vaccines will be evaluated in 250 HIV-1 infected children receiving ART and 125 who are not receiving ART at the time of their primary series of vaccines. These children will also be compared to HIV uninfected infants, born to seronegative (n = 125) and seropositive (n = 125) mothers. Both quantitative and qualitative antibody responses to these vaccines will be evaluated.